When a CCTV system is used to monitor a crowded location, using a large number of cameras, it can be a difficult task to find the current location of an individual person in different images given that the person has been spotted in one image. For example, when a person is seen to perform a crime in a CCTV image at one time, law enforcement officers may find it difficult to find that same person in CCTV images of a crowd minutes later without taking the time to trace the person through a series of images, possibly from different cameras.
Various automated methods have been proposed in the prior art to automate identification of the same person in different images. In images of crowds the identification also involves a comparison of image features of areas in different images to select areas that potentially show the clothing of the same person. An image feature that has been proposed for this type of comparison is a color histogram of an image area that may show a person. A histogram is a set of counts of occurrences of pixel values in the area that lie in respective “bins”, or ranges, in color vector space. More generally, a histogram may be a set of sums of contributions from different pixels. The idea is that such a histogram will remain substantially the same independent of changes of posture and viewing angle, so that comparison of the histogram of an image area where a person has been spotted in one image with the histogram of an image area in another image can be used to detect the person robustly.
Such a method is described in JP 2000-30033. This document proposes to compute a histogram of hue values of pixels in an image area that shows clothing of a person. The use of hue values has the advantage that the histogram is fairly robust against variations of light intensity. By avoiding the use of pixel luminosity the effect of light intensity on the histogram is removed. JP 2000-30033 proposes to select the location of the image area with clothing on the basis of the detected position of another image area that contains a face. The image area that contains a face is detected with the same type of histogram of hue values.
The use of a hue and saturation histogram is mentioned in an article by N. Gheissari et al, titled “Person Reidentification Using Spatiotemporal Appearance”, published in the Proceedings of the 2006 IEEE Computer Society Conference on Computer Vision and Pattern Recognition (CVPR'06). Gheissari et al emphasize the need for invariant signatures to characterize image areas that show clothing. The color values may be normalized to compensate for lighting effects. As an alternative to the use of hue and saturation the use of correction by calibrated image-position dependent brightness transfer functions is mentioned.
However, it has been found that the reliability of person identification by these techniques has limitations.